John O'Boyle/The Star-LedgerCongressman Rush Holt announces his bid for reelection at Princeton Power Systems in Princeton.

TRENTON — U.S. Rep. Rush Holt (D-12th Dist.) is in for a close re-election battle, according to a Monmouth University poll released today.

Holt, who has held the seat for 12 years, leads Republican challenger Scott Sipprelle 51 percent to 46 percent in the survey of 630 likely voters.

This is the toughest political climate Holt has faced in a decade. Although he upset a Republican incumbent to win the seat in 1998 and fended off a tough challenge in 2000, the district was changed to include more Democrats. Since 2002, he has won re-election by comfortable margins.

“This year … he has to contend with the national anti-Democrat, anti-incumbent wave,” said poll director Patrick Murray.

Sipprelle, a millionaire venture capitalist, has also self-funded much of his campaign, allowing him to blanket the district with television, radio and mail advertisements.

The district cuts across central New Jersey, including parts of Monmouth, Middlesex, Mercer, Hunterdon, and Somerset Counties. Support for Sipprelle is strongest in Monmouth County, while Holt polls well in the western part of the district. The battleground is the central portion of the district, which includes a swath of Middlesex County and part of Franklin Township in Somerset County. There, Holt leads Sipprelle 50 percent to 46 percent.

The Monmouth University Polling Institute surveyed 630 likely voters from Oct. 9 to Oct. 12. The margin of error is plus or minus 3.9 percentage points.